The present invention relates to positioning systems for portable radio transceivers, and, more particularly, to a positioning system for a mobile unit of a wireless communications network, based on signals from a satellite network such as the Global Positioning System (GPS) and on signals from the base stations of the wireless communications network.
The Global Positioning System is a system of low Earth orbit satellites that transmit standard signals that can be used to establish the location of a user equipped with a suitable GPS receiver. For civilian applications, the signals are "C/A codes" that are pseudorandom noise (PN) sequences transmitted at a chip rate of 1023 KHz and a repetition period of 1023 chips, so that each frame of the PN sequence includes 1023 chips. Each satellite is assigned its own PN sequence. and the PN sequences of the various satellites are mutually orthogonal.
Superposed on the PN sequences transmitted by a GPS satellite is a Satellite Data Message, transmitted every 30 seconds at a rate of 50 bits per second. Each bit (+1 or -1) of the Satellite Data Message is modulated onto the satellite's signal by multiplying 20 consecutive frames of the PN sequence by the bit. The first 900 bits of the Satellite Data Message includes the satellite ephemeris and time model for the respective satellite. The remaining 600 bits of the Satellite Data Message include a portion of the GPS almanac, which is a 15,000 bit block of coarse ephemeris and time model data for the entire GPS system. In addition, bits 1-8, bits 301-308, bits 601-608, bits 901-908 and bits 1201-1208 of every Satellite Data Message are identical 8-bit (160 millisecond) headers that are invariant in time and that are identical for all the GPS satellites; and bits 31-60, bits 331-360, bits 631-660, bits 931-960 and bits 1231-1260 of every Satellite Data Message are 30-bit (600 millisecond) handover words that are time-variant (these handover words include representations of the time of the week), but that, like the headers, are identical for all the GPS satellites.
Conventionally, a GPS receiver acquires and tracks the signals from at least four GPS satellites, by correlating the received signal with the satellites' respective PN sequences and locking on to the correlation peaks. Once the satellites are acquired and tracked, the GPS receiver decodes the ephemeris and time model, for each acquired satellite, from the respective Satellite Data Messages. These models include sufficient ephemeris data to enable the GPS receiver to compute the satellite's positions. The correlation peaks obtained during the continued tracking of the satellites provide measured times of arrival of these PN sequence frames. The differences between an arbitrary reference time and measured times of arrival, multiplied by the speed of light, are pseudoranges .rho. from the satellites to the GPS receiver. Typically, the reference time is the time at which the satellites commenced transmission of their respective PN sequences, as measured by the GPS receiver clock, which in general is offset from the GPS system clock by an unknown time offset. A pseudorange .rho. is related to the true range R of the respective satellite by .rho.=R+c.sub.b, where the range offset c.sub.b is the time offset. T.sub.0, of the GPS receiver relative to GPS system time, multiplied by the speed of light c: c.sub.b =T.sub.0 c. From these pseudoranges, and from the known positions of the satellites as functions of time, the position of the GPS receiver is calculated by triangulation. Pseudoranges to at least four satellites are needed to solve at least four simultaneous equations of the form EQU .vertline.s-r.vertline.=.rho.-c.sub.b
where s is the position vector of a satellite and r=(x,y,z) is the position vector of the GPS receiver, for the three unknown Cartesian coordinates x, y, z of the GPS receiver and for c.sub.b. The satellites are sufficiently far from the GPS receiver that these equations can be linearized in x, y and z with no loss of accuracy.
Several methods are known for increasing the efficiency with which a GPS receiver establishes its position and for reducing the power requirements of a GPS receiver. Schuchman et al., in U.S. Pat. No. 5,365,450, which is incorporated by reference for all purposes as if fully set forth herein, teach the provision of the Satellite Data Messages to a GPS receiver integrated into a mobile unit, such as a cellular telephone, of a wireless communications network such as a cellular telephone network, by transmitting the Satellite Data Messages to the mobile unit from a base station of the network via the control channel of the network. The Satellite Data Message is 30 seconds long, so even under ideal reception conditions with parallel processing of the signals from all the satellites in view, it necessarily takes more than 30 seconds to get a GPS position fix. Prior knowledge of the Satellite Data Message reduces this time to under 10 seconds.
Krasner, in U.S. Pat. No. 5,663,734, which is incorporated by reference for all purposes as if fully set forth herein, teaches a GPS receiver for a mobile unit in which, as in the GPS receiver of Schuchman et al., the Satellite Data Message is obtained by a wireless link to a base station, but then, instead of processing GPS signals in real time, the GPS receiver stores up to one second's worth of signals (1000 PN sequence frames per satellite), along with the initial time of arrival of the signals, and processes the stored signals. Groups of 5 to 10 frames each are summed and correlated with the PN sequences of satellites expected to be in view, and the resulting correlation functions are added incoherently. The summation over up to 1000 frames boosts the signal to noise ratio by a corresponding amount, and the post facto processing requires much less power than real time processing.
Duffet-Smith et al., in PCT Application WO 99/21028, which is incorporated by reference for all purposes as if fully set forth herein, teach a system and method for locating a mobile receiver ("remote unit") of a wireless communications network. The remote unit and a base unit at a fixed location both receive signals on a control channel from three or more base transceiver stations (BTSs), and both correlate invariant portions of the control channel signals. Compact descriptions of the correlation peaks, such as low order polynomial fits, are transmitted to a central processor by both the base unit and the remote unit. The central processor recovers, from the correlation peaks, the time offsets of the BTS transmissions to the base and remote units. These time offsets are the functional equivalents of GPS pseudoranges. Given the (fixed) locations of the BTSs and the base unit the central processor computes the position of the remote unit, essentially by triangulation, and transmits that location to the remote unit.
A GPS positioning system is predicated on the GPS receiver having a clear line of sight to at least four GPS satellites. This often is not the case in an urban environment. Urban environments commonly have cellular telephone networks installed, so in principle the system taught by Duffet-Smith et al. could be used for locating a mobile transceiver configured as a cellular telephone; but cellular telephones generally are in simultaneous contact with at most two base stations. Sheynblatt, in U.S. Pat. No. 5,999,124, Camp, in PCT Application WO99/61934, and Watters et al., in U.S. Pat. No. 5,982,324, all teach methods of using signals from both GPS satellites and terrestrial BTSs to determine the location of a mobile receiver. These prior art methods generally rely on triangulation, as described above, using signals received simultaneously from a combined total of four or more GPS satellites and BTSs. As noted above, usually, at most two BTSs are "in view" simultaneously; and in a sufficiently cluttered and noisy urban environment, useable signals may be available from only one or two GPS satellites at any one time. These prior art methods also rely on all the BTSs being mutually synchronized, at least with each other if not with the GPS system. Such synchronization is optional under the GSM cellular standard. There is thus a widely recognized need for, and it would be highly advantageous to have, a system and method for locating a mobile unit of a wireless communications network, for example a mobile cellular telephone in a cluttered and noisy urban environment, that derives the location of the mobile unit from signals received sequentially from low Earth orbit satellites such as GPS satellites and from base stations of the communications network, without requiring that the BTSs be mutually synchronized.